Emergency Locator Beacons (ELB) are devices designed to be used as a last resort for people in life threatening situations. Every day there are many examples, some ending tragically, where possession of an ELB would have in the least saved much suffering, and quite often saved a life. The increased use of emergency locator beacon devices also saves the government much time and expense searching for missing persons.
Emergency locator beacons are best utilized in situations where individuals are: injured or lost in the wilderness (i.e., hikers, mountaineers, and skiers), lost or in distress at sea, or involved in an aircraft crash. Conventional emergency locator beacons are not designed for "minor emergencies", such as running out of gas or seeking aid for a flat tire. Typical emergency locator beacon devices are small, approximately the size of a cellular phone. Conventional emergency locator beacon devices typically utilize lithium batteries, which yield at least a five year shelf-life with enough capacity remaining to allow 24 hours of continuous operation.
There are currently two basic types of emergency locator beacons in use. The simplest types of units typically simultaneously transmit on 121.5 MHz and 243 MHz. Emergency locator beacons of this type are required to be carried by all commercial aircraft, all private aircraft registered in the United States and some other countries, and are also a requirement aboard commercial ships.
When an aircraft crashes, an acceleration sensor in the device turns an emergency locator beacon device on. The emergency locator beacon transmits an amplitude modulated downward sweeping tone somewhere in the audio frequency range of 1,600 Hz to 300 Hz with a minimum sweep range of 700 Hz, and a sweep rate of from 2 to 4 sweeps per second. The output power is greater than 75 mw. No other information is broadcast other than the sweeping tone. The same type of beacon is used on boats and ships. The difference is that these types of emergency locator beacon are waterproof, float, and are typically either manually or water activated, rather than impact operated as in the aircraft units.
Detection of the emergency locator beam signal is typically made through satellite or aircraft. There are two varieties of satellites that detect the emergency locator beam signals, one called COSPAS, the other SARSAT. The COSPAS system was the first in operation, and was built by the Soviets. The SARSAT system was developed by NASA, and is operated by NOAA.
Satellites process the 121.5/243 MHz signals in a "bent pipe" fashion. That is, if the satellite hears the beacon, it relays the signal in real time. Therefore, for a ground station to hear the beacon, the satellite must be in a position where it is simultaneously visible to both the ground station and the beacon. It typically takes two satellite passes before a location can be determined. It can take up to two hours before a satellite hears a beacon, and often six or more hours before its position can be triangulated. The location accuracy is on the order of ten miles, versus 1 mile for the more technically advanced 406 MHz digital emergency locator beam units.
All commercial aircraft have a dedicated "guard" receiver that continuously monitors the 121.5 MHz and 243 MHz frequencies. Private aircraft operators are also encouraged to periodically monitor the 121.5 MHz channel while in flight. Therefore, there are a large number of additional monitoring "platforms" available to track emergency beacons, in addition to the satellites.
One difficulty with the 121.5/243 MHz units is a high false alarm rate, which approaches 98 percent. Unfortunately, when an emergency locator beacon unit is activated, there is no information transmitted, other than the sweeping tone, indicating that there may be an emergency. The search and rescue organizations must track down each unit, which is an expensive and time consuming process. The process of determining if an alarm is false can delay rescue in actual emergency situations.
As mentioned above, the second, more technically sophisticated type of emergency locator beacon, operates on a frequency of 406 MHz. Unfortunately, the 406 MHz units, while technically superior, are also much more expensive (typically over $1,000 to around $1,500 vs. approximately $100-$200 for the 121.5/243 MHz units). These 406 MHz emergency locator beacon units offer several advantages over the 121.5/243 MHz emergency locator beacon units. First, the 406 MHz emergency locator beacon units have improved location accuracy due to improved accuracy of the transmitted frequency.
Another advantage of the 406 MHz emergency locator beacon units is improved system capacity. Unlike the 121.5/243 MHz units which broadcast continuously, the 406 MHz emergency locator beacon units transmit a half second burst every fifty seconds. Thus, the 406 MHz emergency locator beacon units are "on the air" a small fraction of the time, making more time available for other beacons.
The 406 MHz emergency locator beacons also offer an improved probability of detection due to higher power. These devices broadcast with 5 watts of power compared to approximately 0.1 watt for the 121.5/243 MHz beacons. However, this difference is not as great as it first appears due to the increased path-loss incurred at the higher frequency. To achieve equivalent free space performance at 121.5 MHz, it is necessary to operate with a power of 0.45 watts. The free space range difference between a 5 watt transmitter operating at 406 MHz, and a 0.1 watt transmitter operating at 121.5 MHz, assuming equal antenna gains and receiver sensitivities at the end of each link is about 2.1:1.
Another advantage to the 406 MHz emergency locator beacons is a lower average power drain because of the lower duty cycle. The average output power of a 121.5 MHz beacon operating at 100 mw is 100 mw. The average output power of a 5 watt beacon operating at a 1 percent duty cycle is 50 mw. The high peak power requirements of the 406 MHz emergency locator beacons require either a special battery, or the use of other techniques to handle the high power peaks. These "other techniques" tend to reduce the efficiency of the device, resulting in a higher average power. Additionally, because it is virtually impossible to use a direction finder to home in on a device with such a lower duty cycle, 406 MHz beacons typically also contain a 121.5 MHz transmitter operating at around 20 mw output to assist in precisely locating the beacon during a rescue. The combination of the lower power efficiency and the use of the secondary beacon minimize any advantage gained due to the lower duty cycle.
The 406 MHz emergency locator beacons also offer better coverage than the 121.5/243 MHz emergency locator beacons, because the 406 MHz emergency locator beacons operate in a store-and-forward mode, rather than a bent-pipe mode. The satellites can hear a 406 MHz emergency locator beacon anywhere, store the location, and deliver the data when in view of a ground station. As mentioned earlier, to relay a 121.5 MHz beacon, the satellite must be mutually in view of the ground station and the emergency locator beacon. This results in areas of the globe where there is no coverage, or situations where there is an increased time until a satellite is in a favorable position.
Finally, the 406 MHz emergency locator beacons allow a unique identification of each emergency locator beacon unit. Each 406 MHz emergency locator beacon transmits a unique digital identification number. This number is correlated to an individual. This greatly assists in handling false alarms (the current reported false alarm rate at 406 MHz is 90 percent). The unique identification number also aids in understanding the situation the individual may be in (or if the alarm is false) because the rescue organization can immediately attempt to contact the beacon's owner for contact during an emergency. This enables the rescue organization to gain additional information, such as where the person is, and what they were doing.
In summary, known emergency locator beacons have very high false alarm rates and/or may be prohibitively expensive. The aforementioned systems do not lend themselves to easy identification of the owner of the device. Further it is desirable to have an emergency locator beacon with enhanced communication features.